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BIG PHYSICS, BIG QUESTIONS –

How to build a gamma-ray laser with antimatter hybrid

By David Shiga

HALF matter, half antimatter, positronium atoms teeter on the brink of annihilation. Now there’s a way to make these unstable atoms survive much longer, a key step towards making a powerful gamma-ray laser.

All the elements in the periodic table consist of atoms with a nucleus of positively charged protons, orbited by the same number of negatively charged electrons. Positronium, symbol Ps, is different. It consists of an electron and a positron orbiting each other (see diagram). A positron is the electron’s antimatter counterpart. Though positively charged like the proton, it has just 0.0005 times its mass. Positronium “atoms” survive less than a millionth of a second before the electron and positron annihilate in a burst of gamma rays.

In principle, positronium could be used to make a gamma ray laser. It would produce a highly energetic beam of extremely short wavelength that could probe tiny structures including the atomic nucleus – the wavelength of visible light is much too long to be of any use for this.

The trouble is that this means assembling a dense cloud of positronium in a quantum state known as a Bose-Einstein condensate (BEC). How to do this without the positronium annihilating in the process was unclear.

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Now a team led by Christoph Keitel of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, suggests that ordinary lasers could be used to slow the annihilation. The trick is to tune the lasers to exactly the energy needed to boost the positronium into a higher energy state, in which the electron and positron orbit farther from one another. That makes them much less likely to annihilate (arxiv.org/abs/1112.1621).

The positronium will eventually lose energy by emitting photons and return to the annihilation-prone state. But the team calculates that about half the excited positronium atoms can survive for 28 millionths of a second on average, 200 times as long as unexcited ones.

This may be long enough to assemble the BEC cloud. In a BEC, positronium atoms behave in lockstep, so when one annihilates itself, the rest follow suit, producing a burst of laser radiation made of gamma rays.

The positronium atoms behave in lockstep – when one annihilates, the rest follow, producing a laser

It may sound like a lot of work, but one thing makes the task easier. Ordinary atoms can only form a BEC when cooled gradually to within a fraction of a degree of absolute zero. By contrast, due to quantum effects, positronium will form a BEC at close to room temperature.

Where mirror, dark and anti-matter meet

Half a century after it was first made, positronium could find uses. As well as powering a gamma ray laser (see main story), it might put the strange theory of mirror matter to the test.

The idea that every particle has an identical – but so far undetectable – mirror partner was dreamed up to explain baffling asymmetries in the emission of electrons from radioactive atoms. Mirror matter has also been touted as a candidate for the mysterious dark matter that makes up 80 per cent of the universe.

The theory says that particles of ordinary matter might very occasionally transform into their mirror-reversed versions, effectively disappearing from view. Positronium normally ends its life by hurling out a flurry of gamma rays. If the mirror world exists, positronium might sometimes turn into mirror matter and vanish without these emissions.

The idea could be tested by trapping positronium in a chamber and keeping track of how much energy it gives off as gamma rays. If the amount is smaller than expected based on the number of positronium atoms that entered the chamber, then some of it may be turning into mirror matter. New calculations by Sergei Demidov of the Institute for Nuclear Research in Moscow, Russia, and colleagues indicate this should happen often enough to be detectable (arxiv.org/abs/1111.1072).